1. Field of the Invention
The present invention relates to a method for producing a lead-base alloy grid for a lead-acid battery, the grid being useful for automotive batteries, VRLA batteries, industrial cycle use batteries, vented batteries and VRLA batteries for standby, cylindrical wound batteries, and the grid having excellent mechanical strength, corrosion resistance, and growth resistance.
2. Description of the Related Art
Recently, as the use of automobile trims has increased and the allowance of useless spaces has been reduced, engine rooms, in which lead-acid batteries for automobiles are placed, have become places of a higher temperature environment than before. In addition, lead storage batteries are always in the state of overcharge, so that they have a shorter life than other lead-acid batteries. Further, the Pb—Ca alloy grid introduced with the intention of obviating the necessity of maintenance has tended to cause the problem of growth, which is deformation of the anode grid by corrosion or elongation, and thus have a shorter life than conventional ones.
These problems such as corrosion and growth can be resolved by decreasing the Ca content in the Pb—Ca alloy substrate, but the decrease of the Ca content results in the decrease of Ca-containing intermetallic compounds such as Pb3Ca and (Pb,Sn)3Ca to cause the deterioration of the grid strength and deformation of the grid during pasting of an active material paste.
Then, it was attempted to decrease the Ca content in a Pb—Ca—Sn alloy, for example, from 0.09% by mass to 0.06% by mass, and then 0.04%, and compensate the loss with Ba or Ag thereby improve the strength. However, sufficient improvement of the mechanical strength was not achieved.
A method for improving the strength of a Pb—Ca—Sn alloy through natural aging is disclosed in R. D. Prengaman, J. Power Sources 95 (2001) 226. It is shown that an alloy containing 0.065% by mass of Ca requires aging treatment for 24 hours, and an alloy containing 0.045% by mass of Ca requires aging treatment for 14 days, and an alloy containing 0.025% by mass of Ca requires aging treatment for 60 days to achieve intended hardness. However, the method requires too much time for natural aging of an alloy containing lower Ca, and is thus insufficient to be practical.
Jpn. PCT National Publication No. 2004-527066 discloses a method for subjecting a Pb—Ca—Sn—Ag alloy containing 0.02 to 0.06% by mass of Ca to artificial aging at 100° C. for 3 hours. WO03/088385A1 discloses a method for subjecting a Pb—Ca—Sn—Ba—Ag alloy containing 0.02 to 0.05% by mass of Ca to heat, treatment at a temperature of 80 to 150° C. for a period of 0.5 to 10 hours within 1000 hours after casting the grid. However, these methods involve a wide range of mechanical variation, and the artificial aging may be ineffective. Therefore, these methods have problems with stability of plant operation.
In order to improve the mechanical strength of a Pb—Ca—Sn alloy grid containing a decreased amount of Ca, the inventors performed differential scanning calorimetry of a Pb—Ca—Sn alloy, and made a detailed investigation of the result. As a result of this, a broad region over a wide range was found in a temperature range lower than the range for known peaks, with this region being likely attributable to the heat generation process. The region is due to the deposition reaction of the precursor to be the deposit nuclear, and the deposit is considered to grow from the precursor as the nuclear.
On the basis of estimation, the inventors conducted a first heat treatment at low temperature thereby promoting the precursor formation, and then conducted a second heat treatment at high temperature to grow the deposit. As a result of this, the resultant Pb—Ca—Sn alloy grid exhibited improved mechanical strength.
Heretofore, artificial aging such as heat treatment, is regarded as accelerated natural aging for slowly depositing intermetallic compounds such as Ph3Ca and Sn3Ca from oversaturated solid solution by cooling after casting. The precursor herein is considered to be equivalent to the GP zone or intermediated phase deposit in an aluminum alloy. However, there is no report evidently showing the presence of the precursor in a lead alloy.